Rheostat and contact arm therefor



May 4, 1954 F. N. SUMRELL RHEOSTAT AND CONTACT ARM THEREFOR 2 Sheets-Sheet 1 Filed April 16, 1951 TaRRaLL N. SUMRELL,

May 4, 1954 F. N. SUMRELL 2,677,742

RHEOSTAT AND coumc'r ARM THEREFOR Filed April 16, 1951 2 Sheets-Sheet 2 FERRHLL N. Sumawv,

BY W

Patented May 4, 1954 RHEOSTAT AND CONTACT ARM THEREFOR Ferrall N. Sumrell, Uniondale, N. Y., assignor to Hardwick, Hindle, Inc., Newark, N. J., a corporation of New Jersey Application April 16, 1951, Serial No. 221,258

2 Claims.

This invention relates in general to rheostats of the type including a dielectric base, a resistance Winding mounted on a dielectric core which is fixedly secured on the base, and means for connecting said resistance winding in a circuit including a contact arm and brush slidable over a track surface on said winding and over a contact element such as a collector ring.

A prime object of the invention is to provide in a rheostat of this type novel and improved means for connecting the resistance winding in an electric circuit, and particularly a novel and improved contact arm and means for mounting the brush thereon whereby the brush can be easily and quickly mounted on and removed from the contact arm.

Another object is to provide a contact arm and means for mounting it on a rheostat base which shall embody novel and improved features of construction whereby the brush can automatically adjust itself to the contact surfaces of the resistance winding and the collector ring, even though portions of the track surface and the collector ring are out of line with each other.

A rheostat of the type contemplated by the invention includes a shaft to which the contact arm is connected and a bearing sleeve rigidly mounted in the dielectric base and in which said shaft is rotatably mounted; and usually saidshaft has a retaining ring or lock ring that is yieldingly held in abutting relation to one end of said bearing sleeve to take up looseness between the parts.

A further object of the invention is to provide a contact arm which shall embody improved spring means both for taking up said looseness between the parts and for maintaining efficient electrical connection between the brush and the collector ring, and for at the same time eliminating all strains on the shaft that might tend to bind the shaft in said bearing sleeve.

Other objects, advantages and results of the invention will be brought out by the following description in conjunction with the accompanying drawings in which:

Figure 1 is a plan view of a rheostat embodying the invention, the operating knob of the rheostat being removed;

Figure 2 is a plan view of the contact end of the rheostat;

Figure 3 is a side elevation of the rheostat shown in Figures 1 and 2;

Figure 4 is a side elevation as seen from the left of the view of Figure 3;

Figure 5 is a sectional view taken on line 5-5 of Figure 4;

Figure 6 is a sectional view taken on line 66 of Figure 3;

Figure '7 shows the contact arm of the rheostat in plan as it would appear when viewed in the direction of the arrows on the line 7-1 of Figure 6;

Figure 8 is a side elevation of the contact arm shown in Figure 7, and

Figure 9 is a cross-sectional view taken on line 9-43 of Figure 8.

In the various views of the drawing, numeral it designates a base formed of a suitable dielectric, high heat-resisting, preferably ceramic material. The base It) includes a central post I l, a flange 12 at one end of the post, and a sector I3 extending outwardly from the post to the periphery oi the flange l2.

A split ring core [4 of dielectric material is provided for the resistance winding It. This core is seated in the recess of the base is formed by the flange [2 around the central post i l, with the ends or" the split core terminating on opposite sides of the sector l3. The core may be secured on the base in any suitable manner but is shown as bonded to the post H by a layer It of cement or fusible ceramic material such as vitreous enamel. Preferably the surfaces of the winding l5 are also coated with vitreous enamel with the exception of the end of the core and the portion of the winding thereon abutting the flange i2 and the portion of the winding at the opposite end of the core which provides a track or contact surface H for connecting the resistance wire in an electrical circuit. The ends of the resistance wire it are electrically connected to terminal clips i8 that are physically connected to the core in any suitable manner, as shown for example in co-pending application of Dominic J. Holleran, Serial No. 212,553, filed February 24, 1951.

The post H of the base it is provided with an axial bore it for accommodating a metal bearing sleeve 2? One end of the bore 59 is enlarged as shown at 21. i he sleeve 20 is threaded at its outer end to receive the clamping nut 22, while the inner end of the sleeve has a head 23 that seats into a correspondingly shaped socket 24 formed in the end wall of the enlarged portion 2| of bore is. The head 23 holds the sleeve in the bore against rotation and with the clamping nut 22 it prevents longitudinal movement of the sleeve.

Clamping nut is screwed tightly against a plate or disc 25 that lies in a recess 26 formed in the outer wall of the base [0 and around the bore l9. Said disc carries a lug 21 adapted to project into a corresponding hole formed in a panel board or .seated against the flange other supporting surface on which the rheostat is to be mounted, whereby when the locking nut 23 is screwed up against the outer surface of the mounting panel the lug 21 holds the rheostat against rotation on the panel.

The sleeve receives the operating shaft having a control knob 3i fastened to one end thereof for rotating the shaft in the sleeve. An annular groove 32 is formed in the shaft where the shaft extends outwardly of the sleeve, said groove having a lock ring 33 fitted into it for abutting against the end of the sleeve to prevent longitudinal movement of the shaft into the sleeve during operation of the rheostat. The shaft carries a pin 34 adjacent to the end wall of the enlarged portion 2! of the bore l9. This pin serves to limit the turning movement of the shaft by striking a lug Sea on the side Walls of a recess I la in the post, the degree of rotary movement of the shaft being predetermined by the width of the lug.

Mounted on the other end of the operating shaft 36 is a sleeve-like head piece formed of dielectric material and having an axial hole passing therethrough for accommodating the shaft. As best shown in Figure 7 the body of the head piece 35 is square in cross-section to fit into a correspondingly shaped opening 38 in a contact arm 31 that is carried by and rotates with the head piece. The shaft 3?) has lugs or projections 3t extending from it tightly wedgingly fitted into notches in the end of the head piece, whereby the head piece and the contact arm carried by the head piece are rotated when the shaft is rotated.

lhe end wall of the post ii adjacent to the head piece 35 is covered by a metallic and flat contact ring or collector ring 39 that has a lug iil bent into the enlarged portion of the bore if) to assist in supporting the ring on the end of the post. As best shown in Figure 6, the contact ring or collector ring 39 has a strip 41 integral With and extending from the rim thereof, said strip being bent to lie along the outer surface of the post H and then bent again to extend out radially from the post and to fit into a recess 32 formed in the bottom Wall of the sector iii. The portion of the strip that extends beyond the sector I3 constitutes an electrical terminal for the sliding contact of the rheostat. It is held firmly in the recess of the sector [3 by means of a bolt as shown at 43.

Figures '7 to 9 inclusive show best how the contact arm 31 is constructed. It is formed from a resilient metal strip and it comprises a circular disc-dike main portion and a radially projecting receptacle 46 adapted to hold the brush 4'! in sliding contact against the flat surface of the collector ring 559 and the track surface I! of the resistance windin it. The opening 36 which has been mentioned above in connection with the description of the head piece 35 is formed in the main portion 45. Part of the metal from this opening and at the sides thereof is bent up to form lips 48 as shown in Figures 8 and 9 for holding the contact arm firmly on its support.

A plurality of leaf spring fingers d9 are arranged around the opening 35 and as illustrated in the drawing these springs may be formed by being cut and bent from the metal of the flat base portion l5. When the various parts of the rheostats are assembled, as seen in Figures 5 and 6, the springs 49 bear against the collector ring thus causing the contact arm to be well 50 on the head piece 4. 35 and furthermore pulling the operating shaft inwardly of the base Ill to hold the lock ring 33 snugly abutting the end of the bearing sleeve 20 and thereby take up looseness between the parts. The tension produced by these spring fingers on the shaft is balanced about the axis of the shaft so that a purely longitudinal pull is exerted on the shaft, thereby eliminating strains that would tend to bend the shaft or cause binding thereof in the bearing sleeve. As the operating shaft is turned, the spring fingers 49 will slide over the contact surface of the collector ring and provide the desired torque and smoothness of operation of the contact arm. These fingers also will maintain said surface in clean and well polished condition for perfect electrical contact between it and the brush 41.

The receptacle 4B for the brush 4'! is formed by bending the metal strip of the contact arm at the locations 5! (Figures 5 and 8) to provide an end wall of the receptacle, while the side walls and the opposite end wall are formed by the upwardly bent cars 52 and extensions 53, respectively. Figure 8 shows that the bottom wall of the receptacle is not parallel with the main portion 45, i. e. the receptacle is slightly bent toward the resistance winding for adding spring pressure to the contact arm incident to flexing of the portion brush 4? consists of a bar preferably rectangular in cross-section and of a metal of good conducting quality, such as for example, copper. When seen in the position shown in Figure the top surface of th brush is curved inwardly toward the center of the brush whereby r only the end portions of said top surface are high enough to contact the collector ring and the resistance winding 15. The bottom surface of the brush is also curved inwardly but in this case the curve is formed to correspond substantially with the curve of the leaf spring 5 3 on which the brush rests freely. This spring is cut and bent from the base of the receptacle 46 and it serves to press the brush against both the collector ring and the track surface of the resistance winding and to function as a pivot on which the brush may, if necessary, oscillate to ride over any irregularities in the track surface.

To remove the operating shaft and the contact arm from the base It of the rheostat, one need merely to remove the control knob and then by pressing the shaft inwardly against the tension of the leaf springs 29, the lock ring 33 may be removed to release the shaft and permit it to be slid out from the sleeve in the bore [9. Assembly of the rheostat involves the same steps as those just described except of course that these steps are performed in a reverse order. After the contact arm has been fitted onto the head piece and the shaft inserted through the opening in the head piece to extend through the metal sleeve 26), the shaft is pressed inwardly against the tension of the springs 49 to allow for easy insertion of the lock nut into the groove on the projecting end of the shaft.

From the foregoing it will be evident that the invention provides for easy and quick assembly and disassembly of the parts thereof and also provides simple and reliable means for constantly yieldingly pressing the brush into contact with the track surface of the winding. Furthermore by the arrangement of this invention, the operating shaft is maintained under constant spring tension and the springs that provide this tension also serve to keep the contact surface of the collector ring in a well polished condition for optimum electrical contact between the moving parts of the device.

While I have shown and described a preferred embodiment of the invention, it should be understood that this is primarily for the purpose of illustrating the principles of the invention and that modifications and changes may be made in the structural details of the rheostat within the spirit and scope of the invention.

I claim:

1. A contact arm for a rheostat comprising a disc-like main portion of resilient material having an opening and a lateral extension, there being spaced spring fingers stamped up and projecting from said main portion at one side thereof and spaced from each other in encircling relation to said opening, said lateral extension being bent to form portions angularly disposed to the general plane and at one side of said disclike main portion so that said extension is oifset from said plane and a brush-holding receptacle is provided at one side of said extension, said extension also having a spring finger on the bottom wall of said brush-holding receptacle for rockably mounting a brush in said receptacle.

2. In a rheostat having a base, a bearing sleeve fixedly supported in said base, an operating shaft extending through said sleeve, cooperating elements on said shaft and said bearing sleeve to limit longitudinal movement of said shaft in one direction, spring means to yieldingly urge said shaft in said direction, a contact ring on said base concentric with said shaft, an insulating sleeve fixed on said shaft, a contact arm fixedly mounted on said insulating sleeve to rotate therewith and comprising a disc-like main portion of resilient material having an opening and a lateral extension bent to form portions angularly disposed to the plane of said main portion so that said extension is offset from said plane and a brushholding receptacle is formed on one side of said extension, at least one spring finger on the bottom wall of said brush-holding receptacle, a resistance winding on said base, and a brush mounted in said receptacle in engagement with said spring finger and yieldingly rockably held in sliding contact with both said winding and said contact ring.

References Cited in the file of this patent UNITED STATES PATENTS 

